This invention relates generally to the manufacture of integrated circuitry, and more particularly, to techniques for the measurement of junction depths in integrated circuits. Integrated circuits include multiple layers of materials having differing physical and electrical properties. The materials may be doped with small concentrations of impurities to provide p-type or n-type semiconductive regions. The spatial position of a junction between two layers of different type is often highly critical to the operating characteristics of the resulting device, and various techniques have been devised to determine the depth of such a junction.
Since the first development of integrated circuitry, electronic components have been packed onto single substrates in ever increasing densities. In fact the packing density obtainable in integrated circuitry has increased by a factor of approximately four every two years. These higher densities have required closer junction spacings, and the need for a fast and accurate method of determining shallow junction depths has become increasingly acute.
A traditional method for junction depth measurement involves angle lapping and staining a semiconductor wafer, then measuring the distance of interest with an interference microscope. Basically, a wafer is angle-lapped at a relatively shallow angle with respect to its surface, thereby exposing the junctions between adjacent layers. The materials can be contrasted by appropriately staining the exposed cross section. Then the depth of any junction is measured along the lapped cross section, and the perpendicular depth is obtained mathematically from the measurement and the known lapping angle. This technique is described fully in a paper by C. P. Wu et al. entitled "Techniques for Lapping and Staining Ion-Implanted Layers," published in the Journal of the Electrochemical Society, Vol. 126, p. 1982 (1979), and permits the determination of junction depth to an accuracy of .+-.200 .ANG.. In accordance with this angle lapping approach, a massive lapping jig must be employed, to eliminate rocking and insure a planar lapped surface. Also a material such as silicon dioxide (SiO.sub.2) must be deposited on the silicon wafer surface prior to lapping, to avoid edge rounding of the surface.
More recently, T. T. Sheng et al. have developed a method of delineating shallow junctions in silicon structures using transmission electron microscopy, as described in their paper entitled "Delineation of Shallow Junctions in Silicon by Transmission Electron Microscopy," published in the Journal of the Electrochemical Society, Vol. 128, p. 881 (1981). Although this technique exhibits a high degree of accuracy and reproducibility, it requires a relatively lengthy time for preparation.
In another technique, the junction depth is determined by forming a shallow groove across the wafer surface with a cylindrical grooving tool, and measuring the width of the exposed junction and the width of the entire groove. In accordance with a simple mathematical relationship pointed out by B. McDonald et al in a paper entitled "Measurement of the Depth of Diffused Layers in Silicon by the Grooving Method," published in the Journal of the Electrochemical Society, Vol. 109, p. 141 (1962), the perpendicular depth of the junction can be calculated from these two measured distances and the known radius of the grooving tool. Major sources of error arise from the assumption of a perfectly cylindrical groove, and from the difficulty in exactly measuring the width of the groove. Moreover, for very shallow junctions the groove must also be very shallow, and the width of the exposed junction may be very close to the width of the entire groove. Accordingly, the possibility of error is greatly increased.
It will be appreciated from the foregoing that there is still an increasing need for a fast, accurate and reliable technique for measuring junction depths in integrated circuits, particularly circuits of high component density fabricated in very-large-scale integrated structures. The present invention fulfills this need.